It is known that the discharge in electric discharge laser chambers can cause pressure wave disturbances that interfere with subsequent pulses. Laser chambers with provisions for minimizing these disturbances are described in U.S. Pat. No. 5,978,405 which is incorporated herein. The '405 patent is assigned to the assignee of the present invention. FIG. 1 is a drawing of the cross section of a typical KrF excimer laser chamber. The gain region of the laser is a discharge region with a cross section of about 20 mm.times.4 mm shown as 34 in FIG. 1 with a length between elongated electrodes 36A and 36B of about 70 cm. In the chamber laser gas is circulated by fan 38 and cooled by heat exchanger 40. Also shown in FIG. 1 are main insulator 42, anode support bar 44 and preionizer rod 46.
An important use of electric discharge lasers such as KrF excimer lasers is as light sources for integrated circuit lithography. In these applications, the lasers are line narrowed to about 0.5 pm about a desired "center-line" wavelength. The laser beam is focused by a stepper or scanner machine onto the surface of a silicon wafer on which the integrated circuits are being created. The surface is illuminated with short bursts of laser pulses at pulse rates of about 1000 Hz or greater. Very precise control of wavelength and bandwidth are required to permit the production of extremely fine integrated circuit features. The operators of most stepper and scanner machines in use today operate the laser light source at about 1000 Hz, but 2000 Hz sources are being shipped and lasers with even higher repetition rates are being developed. The typical laser gas for the KrF laser is about 99 percent neon at 3 atmospheres and at a temperature of about 45.degree. C. At this temperature a sound wave travels about 47 cm between pulses at 1000 Hz, about 23.5 cm between pules at 2000 Hz and about 11.7 cm at 4000 Hz. Integrated circuit manufacturers desire to be able to operate their laser at any pulse rate within the operating range of the laser while maintaining beam parameters including target wavelength and bandwidth within desired specifications.
Distances between the discharge region of a typical lithography excimer laser and major reflecting surfaces within the laser chamber range from about 5 to 20 cm. Distances between reflecting surfaces in planes perpendicular to the length of the discharge region are mostly between about 5 cm to about 10 cm. Therefore, as demonstrated by a comparison of FIG. 2A showing distances traveled by sound with FIG. 1, a typical discharge created pressure wave traveling at the speed of sound in the FIG. 1 laser operating at 1000 Hz would have to make several reflections in order to arrive back at the discharge region coincident with the next discharge. At pulse rates in the range of 2000 Hz and higher, the pressure wave traveling at the speed of sound may return to the discharge region coincident with the next pulse after only one reflection.